Field of the Invention
This invention relates to antennas and more particularly relates to antennas operative at two (2) widely separated frequencies. The antenna of this invention is particularly adapted to be used in a transponder which constitutes a tag attached to an object to identify the object by transmitting to a reader signals identifying the object.
As commerce becomes increasingly complex, the volume of products requiring individual identifications increases. For example, containers holding goods are stacked on merchant ships. When the merchant ships reach a destination port, only individual ones of such containers have to be unloaded and the remaining containers may be retained on the merchant ship until subsequent destination ports are reached. It would be desirable to identify, on a remote basis such as in the order of thirty (30) to forty (40) feet, the containers which have to be unloaded at the destination port. By identifying such containers on a remote basis, any need for merchant seamen or longshoremen at the destination port to have to inspect such containers individually may be eliminated.
Systems have been developed for identifying an object on a remote basis. Such systems include a reader displaced from the object for interrogating a transponder at the object. The transponder has an identifying code which is individual to the object being interrogated. This code is represented by a sequence of binary 13 s and binary 0's in a pattern individual to the object. Each of the binary 1's and binary 0's in this sequence is converted to a plurality of signals which are transmitted to the reader. The signals in each plurality may have first and second frequencies in a particular pattern to identify a binary "1" and may have first and second frequencies in another pattern to identify a binary "0".
The transponder has an antenna (or antennas) for transmitting the identifying signals to the reader. A problem exists in the transponder with respect to the antenna because the signals are transmitted at different frequencies in different parts of the world in accordance with Governmental standards adopted in such different parts of the world. For example, the transmitting frequency adopted by Government regulations in the United States, Europe and Hong Kong has been approximately nine hundred and fifteen megahertz (915 MHz). The transmitting frequency adopted by Government regulations in the Far East (except for Hong Kong) has been approximately twenty four hundred and fifty megahertz (2450 MHz).
A considerable effort has been made, and a significant amount of money has been expended, to provide a single transmitting assembly, including a single antenna, which is able to receive and transmit signals at each of the two (2) frequencies specified in the previous paragraph. In spite of such effort and such money expenditure, a satisfactory antenna assembly has not been provided to the present time to meet the above requirements.
This invention provides a transmitter assembly which is useful in a transponder to receive and transmit signals at a first frequency such as approximately nine hundred and fifteen megahertz (915 MHz) and at a second frequency such as approximately twenty four hundred and fifty megahertz (2450 MHz). The transmitter assembly includes a single antenna assembly defining two (2) antennas each disposed on a single dielectric member and each operative at an individual one of the frequencies. Each of the antennas is effective in receiving and transmitting signals at its individual frequency.
In one embodiment of the invention, a dielectric member may be thin and planar and may have first and second opposite surfaces. An electrically conductive material is disposed on the first surface at one end of the first surface and an electrically conductive material is disposed on the second surface at the opposite end of the second surface. The conductive materials in the first and second surfaces define a first antenna operative at a first frequency such as nine hundred and fifteen megahertz (915 MHz).
Slots are provided in the conductive material in the first surface. The slots define a second antenna operative at a second frequency greater than the first frequency. This second frequency may be twenty four hundred and fifty megahertz (2450 MHz). The slots include first and second slots extending in a direction transverse to the relative direction of the conductive materials on the first and second surfaces. The first and second slots may have substantially equal lengths and may be aligned with each other. The slots also include third and fourth slots extending in such relative direction. The lengths of the first and second slots define the frequency of the signals from the second antenna and the lengths of the third and fourth slots define the impedance of the second antenna. The third and fourth slots are disposed in a spaced and parallel relationship to define a conductive portion.
Additional conductive material is disposed on the first surface of the dielectric member in electrical communication with the conductive portion on the first surface. The additional conductive material is disposed opposite the conductive material on the second surface and is provided with a length defining the impedance of the first antenna.